Many devices such as automobiles, boats, motorcycles, agricultural machines, and some spare power supply systems use a battery which is rechargeable. Sometimes, the electric power stored in these rechargeable batteries is depleted, and the battery must be recharged. Battery chargers which are used to recharge a battery are well known devices. Such chargers supply direct current to the positive terminal of an energy depleted battery. Generally, the battery chargers include an alternating current ("AC") to direct current ("DC") converter for generating a direct current from an AC household power supply. The chargers also include red and black attachment cables for connecting the charger to the depleted battery. When connecting the charger to the battery, the red cable must be connected to the positive terminal of the battery, and the black cable must be connected to the negative terminal of the battery or to a ground so that the direct current is supplied to the positive terminal of the battery.
If the red and black cables are reversed and not properly connected to the battery terminals, then the battery charger will supply the direct current to the negative terminal of the battery. This situation can result in overheating of the battery charger, excess arcing between the connection cables and the terminals of the battery, and, in extreme cases, battery explosions.
Currently, there are battery chargers which are able to detect polarity (e.g. U.S. Pat. No. 4,876,496 to Duncan). However, present devices can only detect polarity if the battery has a minimum voltage greater than 7.2 volts and are not effective in determining polarity of a depleted battery having a voltage of less than 7.2 volts. Clearly, a polarity sensing feature in a battery charger which can sense the polarity for a battery having any voltage greater than zero volts would protect over a greater range of situations.
Also, there are batteries which cannot be recharged because the battery has been (1) sulfated; (2) previously, negatively charged; or (3) frozen. These batteries need to be replaced. Currently, battery chargers do not have the capability to determine if a battery is accepting any charge. Without such a feature, hours of electrical energy may be wasted trying to charge a "dead" battery. A battery charger which can determine whether a battery can accept charge is desirable.
Also, currently, battery chargers continue to supply current to the positive terminal of a battery as long as the battery charger is connected to an AC outlet and the battery is connected to the battery charger. Overcharging a battery can damage the battery or limit its life. Thus, it is important to know when the battery has been fully charged. Moreover, in order to save energy, it would be more efficient if the battery charger would stop supplying current once the battery has been fully charged. Thus, a battery charger which will stop supplying current to the battery when it is fully charged is desirable.
Finally, some battery chargers do determine whether a battery is fully charged. However, these chargers make this determination based on detecting a decrease in the rate of change in voltage over time. This method is flawed for two reasons: (1) a slow change in voltage could be indicative of a frozen or negatively charged battery; and (2) this method does not ensure that the battery is fully charged. A battery charger which can determine if a battery is fully charged before turning off the current would be useful.